Among plated steel materials most commonly used as having a good corrosion resistance, there are galvanized-type and aluminum-plated-type steel sheets. Such plated steel sheets are used in various manufacturing industries including the fields of automobiles, home electric appliances and building materials. In addition to those, plated steel materials are used in various fields including plated steel wires and hot dip plating. In particular, steel materials to which Al-added plating is applied have a high corrosion resistance and, therefore, have been increasingly used in recent years.
To improve the corrosion resistance of such galvanized-type steel sheets, the use of Zn—Al—Mg—Si hot dip plated steel sheets has been described in Japanese Patent No. 3179446. Further, in Japanese Patent Publication No. 2000-064061, it was described that a painted steel sheet more excellent in corrosion resistance can be obtained by adding one or more elements of Ca, Be, Ti, Cu, Ni, Co, Cr and Mn to such a proposed Zn—Al—Mg—Si hot dip plated steel sheet.
Further, Japanese Patent Publication No. H5-125515 describes that, when Ti is added to a Zn—Al hot dip plated steel sheet with the aim of improving the corrosion resistance of a galvanized-type steel sheet, the steel sheet is excellent in resistance to black discoloration with age. Furthermore, Japanese Patent Publication No. 2001-295015 describes that a surface appearance is improved by adding Ti, B and Si to a Zn—Al—Mg hot dip plated steel sheet.
However, with the aforementioned and with other disclosed plated steel sheets, surface smoothness is likely insufficiently secured.
In the case of a Zn—Al binary alloy, the eutectic point thereof is 6% Al-94% Zn in mass and, when an Al concentration is higher than that, an Al phase crystallizes as a primary crystal. Meanwhile, in the case of an Al—Si binary alloy, the eutectic point thereof is 87.4% Al-12.6% Si in mass and, when an Al concentration is higher than that, an Al phase crystallizes as a primary crystal.
In the case of a Zn—Mg—Al ternary alloy, the ternary eutectic point thereof is 3% Mg-4% Al-93% Zn in mass and when an Al concentration is higher than this an Al phase crystallizes as a primary crystal. When a solidification speed of plating is sufficiently secured at the time of hot dip plating, the plating solidifies before an Al phase grows large and therefore surface smoothness does not deteriorate. In contrast, when a solidification speed of plating is low, the problem is that an Al phase grows large at first, causing ruggedness to form on a plated surface and, as a result, the surface smoothness deteriorates.
Nevertheless, the technology described in the aforementioned Japanese Patent No. 3179446 generally does not take the problem of the deterioration of surface smoothness into consideration. Further, though the technology described in the aforementioned Japanese Patent Publication No. 2000-064061 may employ the addition of one or more elements of Ca, Be, Ti, Cu, Ni, Co, Cr and Mn with the aim of improving post-painting corrosion resistance, the technology neither takes the problem of the deterioration of surface smoothness into consideration, nor refers to an intermetallic compound. The technology described in the aforementioned Japanese Patent Publication No. H5-125515 does not take the problem of the deterioration of surface smoothness into consideration. Furthermore, though the technology described in the aforementioned Japanese Patent Publication No. 2001-295015 employs the addition of Ti and B with the aim of suppressing the formation and growth of a Zn11Mg2 phase that deteriorates surface appearance, the technology neither takes the problem of the deterioration of surface smoothness into consideration nor refers to an intermetallic compound.